my husband is filing for my three children and so far my son is approved i need help with the visa centre how can i avoid paying online fees affidavit

You will need to contact the National Visa Center and explain why the fee cannot be paid online. The National Visa Cebter will then make arrangements for the payment of the fee through other means. You can read more at https://travel.state.gov/content/visas/en/immigrate/immigrant-process/documents/fees/nvc-fee-payment-faqs.html. It would be best for you to pay the fees online. Maybe a friend or family member can help. You can read more about family immigration at http://myattorneyusa.com/family-immigration.